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PWFA Experiments in Static Fill Argon

PWFA Experiments in Static Fill Argon. Some numbers issues and mysteries. Ken Marsh 5/30/2013. M1. How did we get 20-30 GeV gain when we should be very limited by head erosion?. L HE ≅ σ z /v etch. Comparing Li to Ar ionization potentials. Li = 5.4 eV , Ar = 15.8 eV ,

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PWFA Experiments in Static Fill Argon

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  1. PWFA Experiments in Static Fill Argon Some numbers issues and mysteries Ken Marsh 5/30/2013

  2. M1. How did we get 20-30 GeV gain when we should be very limited by head erosion? LHE ≅ σz /vetch Comparing Li to Ar ionization potentials Li = 5.4 eV,Ar = 15.8 eV, relative reduction in HE length (15.8/5.4)1.73 = 6.4 20 GeV in 12 cm is 170 GeV per meter!

  3. M2. Sub 10 um beam size required to tunnel-ionize argon,but For typical beam parameters β and ε, Calculated beam size σ = (βε)1/2 = 10 to 20 um Measured σr was larger than 20 um How was full Ionization achieved?

  4. Some basic parametersfor 20 Torr Argon Density Range 10% ionized 7x1016 100 % ionized 7x1017cm-3 Normalized beam parameters For σr = 10 μm, kpσr = .5 to 1.5 For σz = 20 μm, kpσz = 1 to 3

  5. M3. Expected blow out radius is greater than the ionization radius Based on the maximum radial electric field of the e-beam, the estimated tunnel ionization radius is ≥ 1.6σr Normalized blow out radius Blow out radius/ionization radius ≅ Rb/1.6kpσr ranges from 3.75 to 1.25 Thus favoring higher density plasma!

  6. M4. Beam Mismatch Assuming no density ramp up to reduce the beam size and improve the match, there is a severe mismatch, especially at higher plasma density. For ne = 4x1017 cm-3β = .2 cm. The FACET betas were 10 cm and 100 cm. Betatron motion and X-ray generation hνc = 32 MeV Power radiated ~ n2r2 Divergence kbr Total counts on Lanex ~r2

  7. M5. Helium Ionization and Trapping • Ionization was observed in pure helium • IP Helium 24.6 eV, IP Argon 15.8 and 27.6 eV • Is ionization trapping from ArII possible? ArIII?

  8. M6. Smelled like sulfur after opening vacuum • Photo spallation argon? S hν Ar α

  9. Summary Unexpected Results • High Energy gain despite limits by HE • Results, in theory, favor high density plasma formation • Without density ramp, beam is severely mismatched. Consider time like density ramp. • In all cases, results favor full ionization with radius of ionization > Rblow • How is full Argon ionization achieved? • And some ionization of Helium?

  10. Why you should not do PWFA experiments with bounded gas volume

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